Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a first image of a display screen or portion thereof with a transitional graphical user interface according to the claimed design; and,

FIG. 2 is a front view of a second image thereof.

The appearance of the transition is sequential from FIG. 1 to FIG. 2. The process or period in which an image transitions to another image forms no part of the claimed design.

The gray regions represent contrast.

The broken line showing a display screen or portion thereof forms no part of the claimed design. The broken lines showing portions of the graphical user interface form no part of the claimed design. The broken lines showing an electronic device is for environmental purposes and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 